The present invention relates to a system and method for minimizing torque increases causes by high-speed engine operation.
Camless engines use a direct electro-mechanical actuation of exhaust and/or intake valves of the engine without reliance on a mechanical camshaft. As a result, the cylinder air charge can be controlled by opening and closing valves at will, providing benefits such as increased fuel economy due to reduced pumping losses, emission reduction due to the internal exhaust gas recirculation, improved low end torque, and improved transient response.
Unlike mechanically driven cams in engines, where the throttle is used for charge and torque output control, intake valve closing (IVC) timing is typically adjusted to achieve the desired engine torque output. If the torque demand is getting low, the engine demand for air is also getting low and IVC timing is adjusted to force the intake valve to close earlier.
However, there is a disadvantage with such an approach. In particular, at high engine speeds and low torque demand conditions, the time required between opening and closing of the intake valve may become excessively small so that the actuators (that have a finite speed of response) can no longer deliver it. At these conditions, late intake valve closing strategy (with valve closing past BDC in compression stroke) can typically employed. In LIVC, more charge is drawn in than needed and then the excess is pushed back into the intake manifold by upward piston motion.
The inventors of the present invention have recognized a disadvantage with such an approach. Typically, the transition between early intake valve closing (EIVC) operation and late intake valve closing (LIVC) operation as may be required as a result of the driver torque demand drop at high engine speeds is difficult to manage. When the air-fuel charge starts to be pushed back into the intake manifold from the cylinder, it disturbs the air-to-fuel mixture in the intake ports and affects in a difficult to predict way the fuel evaporation and wall-wetting characteristics. Thus, deviations in the exhaust air-to-fuel ratio, with the negative impact on catalyst performance and tailpipe emissions, may occur.
The above disadvantages are overcome, by a system comprising: an engine having at least an intake valve in a cylinder of said engine; a valve actuation unit coupled to said intake valve; a torque transmitting unit having at least a first and second torque transmission path coupled to said engine, with said at least two torque transmission paths having a clutch that affects torque of said respective path; and a control unit for controlling said torque transmitting unit and said valve actuation unit, said controller providing an indication of an engine torque output disturbance caused limitations in minimum opening of said intake valve, and adjusting a clutch parameter of at least one of the first and second clutches based on an said engine torque output disturbance so that a vehicle drive torque is substantially unaffected by said torque disturbance.
By using such a system, the engine torque output can be temporarily maintained at levels higher than the actual engine torque demand to enable the engine to continue in EIVC operation instead of transitioning into LIVC mode. In other words, the difference between the engine torque demand and actual torque output is managed by providing multiple transmission paths. At the same time transient air-fuel ratio excursions caused by EIVC to LIVC transitions and negative impact on emissions can be avoided. Note also that the need for LIVC operation occurs mostly during transients and is for short periods of time and, as such, the present invention is well suited for such an application.